Breaking the pocket enigma

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+
+# Breaking the Pocket Enigma
+
+![centre-aligned Pocket Engima](pocket-enigma-small.jpg)
+
+Using cribs
+
+---
+
+# Breaking the Pocket Enigma
+
+A _crib_ is a piece of plaintext we believe to be in the enciphered message.
+
+This is the way Enigma messages were broken in WWII.
+
+There are many keys, and doing the naÃ¯ve Bayes analysis was too expensive.
+
+The possible keys were filtered by finding the ones that could have produced the crib. These filtered keys could then be checked manually.
+
+At Bletchley Park, the filtering was done by the Bombes.
+
+---
+
+# Breaking by cribs
+
+```
+Given a message, a wheel spec, a crib, and the location of that crib:
+    Go through each key in turn
+    If the plaintext matches the crib:
+        Add it to the list of possible keys
+Return the possible keys
+```
+
+## Example
+```python
+pocket_enigma_break_by_crib('kzpjlzmoga', 1, 'l', 3)
+['a', 'j', 'n']
+```
+
+Using wheel 1, the `j` (`ciphertext[3]`) could become `l`  if the wheel starts the message on a, j, or n.
+
+

+ + + + + + +